In continuous casting, molten steel is poured from a ladle into a tundish, and further, this molten steel is poured into a mold. A solidified shell forms along the outer circumferential part of the molten steel in the mold, and a cast slab in this state (the solidified shell and the molten steel inside the solidified shell) is withdrawn beneath the mold. After that, the cast slab is solidified to the inside by secondary cooling in a spray zone. The cast slab obtained as described above is cut into proper sizes. If necessary, the cast slab is adjusted to proper temperature by bloom reheating, and after that, blooming is carried out thereon.
Cracks appear in surfaces of the cast slab upon bloom reheating according to cooling conditions for the cast slab. Therefore, methods for cooling cast slabs are figured out in order to prevent such cracking. For example, for the purpose of refining the structure of the outer layer of a cast slab, the cast slab is cooled (tertiary cooling) after being cut, using a bloom cooler that is a cooling device outside a continuous casting machine.
Patent Literature 1 describes that after being cut into prescribed lengths, the bloom cast by a continuous casting is cooled from the temperature range just above Ar3 point by using a bloom cooler. According to Patent Literature 1, the bloom is cooled by controlling the water quantity density of the upper surface of the bloom that is horizontally placed to 5×10−4 to 4×10−3 m3/sm2 (=30 to 240 L/min/m2), and the water quantity density of the side surfaces thereof and the lower surface thereof are differentiated from that of the upper surface thereof, so that cracks appearing at the time of cooling the bloom can be prevented.
Patent Literature 2 describes that when cooling a bloom at a temperature right above the Ar3 point by using a bloom cooler, the transfer velocity of the bloom is made to be 3 to 10 m/rnin. According to Patent Literature 2, whereby, the bloom is cooled in a manner that the bottom side of the bloom is evenly cooled.
Each method of Patent Literatures 1 and 2 is intended for the existence of a structure where γ grains are refined in the outer layer of the bloom at the time point when bloom reheating is carried out.
On the other hand, in Patent Literature 3, secondary cooling of quenching of the cast slab is performed, and whereby the structure of the outer layer of the cast slab is reformed to that of high hot ductility, to obtain the cast slab having no cracks on the surfaces.